A wireless communication system using IEEE802.11a/n shares a frequency bandwidth of 5 GHz with radar such as a weather radar or a terrain radar. A wireless communication apparatus in the wireless communication system checks arrival of radio wave from the radar in order to avoid interference. If the wireless communication apparatus detects the radio wave from the radar in a frequency bandwidth, the wireless communication apparatus may abstain from using the frequency bandwidth. There are two radars in the 5 GHz band, that is a pulse radar and a chirp radar. A transmission time duration of the pulse radar is usually shorter than that of a frame of the wireless communication system. Accordingly, the wireless communication apparatus is able to distinguish between the pulse radar and the frame of the wireless communication system by measuring a power of a receiving wave in terms of time. On the other hand, the transmission time duration of the chirp radar is almost same as the frame of the wireless communication system. Therefore, it is difficult for the wireless communication apparatus to distinguish the chirp radar and the frame of the wireless communication system by only measuring the power of the receiving wave. In order to distinguish the chirp radar and the frame of the wireless communication system, the wireless communication apparatus may need to analyze a wave form.
A conventional wireless communication apparatus inputs the receiving radio wave into a demodulation unit of IEEE802.11a/n in order to distinguish the chirp radar and the frame of the wireless communication system. If the conventional wireless communication apparatus can not demodulate the receiving radio wave correctly in the demodulation unit, the conventional wireless communication apparatus determines that the receiving radio wave is the chirp radar.
In other conventional wireless communication apparatus, if the conventional wireless communication apparatus has a replica of a wave form of other wireless communication system (for example, HiperLAN), the conventional wireless communication apparatus calculates correlation between the receiving wave and the replica of the other wireless communication system. If the correlation is low, the conventional wireless communication apparatus determines that the receiving wave is the chirp radar.
However, the conventional wireless communication apparatus may falsely determine that the receiving wave is the chirp radar, if the conventional wireless communication apparatus receives the frame of other wireless communication system degraded severely by radio channel or an unknown frame of a new wireless communication system. For example, a green field packet of IEEE802.11n is the unknown frame for a wireless communication apparatus using IEEE802.11a. In order to reduce mistake of the determination, the wireless communication apparatus is better to measure a time variation of a frequency which is unique feature of the chirp radar.
One of the techniques for detecting the time variation of the frequency is disclosed in JP-A 2007-166611 (KOKAI). In this reference, a wireless communication apparatus estimates frequency of the radio wave by counting number of zero crosses of the radar wave for a duration. The wireless communication apparatus estimates the frequency of the radio wave in each of continuous three periods. If the frequency of the radio wave varies linear, the wireless communication apparatus determines that the receiving radio wave is the chirp radar. However, the wireless communication apparatus may falsely determine, if at least one of the estimated frequency includes a large error.
Accordingly, an advantage of an aspect of the present invention is to provide a wireless communication apparatus which detects the chirp radar more certainly.